Dear MrsN,

Thanks for the comments. I'm glad you find the blogs useful.

With any heat pump, insulation is the key and we would always recommend that the maximum amount of insulation is fitted. Without insulation the heat pump has to operate at a higher temperature and becomes less renewable as it uses more electricity. In the worse case you could end up with high energy bills and cold rooms. For any heat pump insulation we would recommend levels of insulation to at least current Building Regs.

If you are considering an air source make sure that the unit will provide the correct output at the higher internal temperature and low ambient temperature outside. A number of manufacturers state the outputs at 35C internal and 7C external. Deviation from these conditions will decrease the output of the unit.

You will also need planning permission for the air source and take into consideration defrost cycles, which either use direct immersion or take heat out of the building (from a buffer vessel).

With regards to running oil upstairs and a heat pump downstairs, it is possible to do this, however you need to ensure that the heating systems are completely separate (otherwise with its high return temperature the oil boiler will take all the load, and the heat pump will never turn on) and both heating systems are operated at the same time. If they aren't then the air source needs to be sized for the whole property as it will try and heat the whole building.

Finally with regards to the underfloor and running this first on oil, just make sure that the underfloor is designed for a heat pump. This generally means closer centres on the pipes due to the lower flow temperature from the heat pump. You might also need to close any mixing valves on the system as for a heat pump these are not required but are generally fitted for an oil boiler.

John Lightfoot who also writes on this website is a good contact for air source. My company Kensa only manufacture ground source as we feel this is a better solution.

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Hi John,
Can I ask some really basic questions?  I have read the blogs here which are very helpful & so far so good.  We have just had an offer accepted on a mid-1800s cottage needing complete renovation which currently runs oil fired CH/HW & I am desperate to reduce our dependence on this.  There is also a damp problem as the floors consist of bricks & then earth so we have been advised to dig out put in concrete screed/insulation etc.  So, the obvious answer seems to be underfloor heating throughout ground floor with an air source heat pump.  Upstairs and HW would still run on oil.  QUESTIONS:
Does this sound all sound logical & feasible to you?
In terms of the 'sufficient insulation' required would good loft insulation, secondary glazing & brick walls be likely to be sufficient?
Lastly, as we probably can't afford to do all the work/secondary glazing in one go, have you ever heard of underfloor heating being fitted & running off oil initially then an air source heat pump being retro-fitted to the system a year later once all the insulation was in place?

Thanks for your advice & sorry for long-winded questions!
MrsN

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Hi Bob,

Thanks for the comment. Its interesting to see that you have found what we have always thought written in a book! Our general stance with clients now is for existing installs either to

a) if they haven't yet installed, reduce their boiler temperature to around 50C and see if the radiators provide enough heat.

b) if they have committed to a heat pump, leave the existing radiators and only replace if the room feels cold.

c) have a radiator survey done.

A number of our clients are finding that they are warm enough on their existing radiators, simply due to the original heating engineer oversizing them.

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In my research into heat pumps, I found a comment in one of the older books "Heat pump" by McMullan - there are hardly any new ones! - that radiators are likely to be oversized on existing systems. I quote

"Boilers are cheap, and it is customary to install quite large boilers in quite small buildings, to ensure that there will always be enough heat. As an example, a house of 100 m2, which in typical cold winter weather might require 5-7 kW of heating, might easily be fitted with a boiler of 18 kW ... output. This means that enough radiator area has to be provided to emit 18 kW at reasonable water temperatures. ... Consequently it is necessary to install radiators with enough area to dissipate only about 7 kW at 50/60°C, which by a happy chance is almost exactly 40% of 18 kW." (McMullen, 1981, p107) 

This implies that (probably older) radiators already installed in houses would be sufficiently large enough to provide enough output from a hp at 45-55 degrees. Does anyone have any experience of this?

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